This invention relates to the field of integrated circuit fabrication. More particularly, the invention relates to detecting an end point in a planarization process, such as an electrochemical planarization process.
Generally, the fabrication of an integrated circuit includes the formation of conductive structures within the various layers of the circuit These structures include metal vias, which provide a conductive path from one layer to another, and metal traces, which connect devices within the same layer.
The processes for forming such conductive structures generally involve a metal deposition step such as electroplating, chemical vapor deposition, or physical vapor deposition, and a planarization step such as chemical mechanical polishing or electrochemical planarization to remove unwanted portions of the metal applied during the deposition step, thereby leaving the metal only in the vias or trenches to form interconnects. More recent processes, such as electrochemical mechanical deposition, combine the deposition and planarization steps such that much of the unwanted metal is removed while the desired metal structures are forming.
A critical factor in practically any metal planarization process is determining when the process should end. For example, if a polishing process continues for too long a time, the polishing removes some of the metals in the vias or trenches, which were not intended to be removed. If the polishing process continues for too short a time, the polishing will not completely remove the portions of the metal layer that were intended to be removed.
Currently, determining the end point of a planarization process is based at least in part upon assumptions made about the thickness and hardness of the material to be removed, and the abrasive properties of the polishing pad and polishing slurry. If any of these assumptions are inaccurate, then the end point of the planarization process will not be determined correctly.
What is needed, therefore, is a system for accurately detecting the end point of a planarization process.
The above and other needs are met by an apparatus for determining an end point of an electrochemical planarization process for removing a metal from a surface of a substrate submerged in an electrolytic solution. The apparatus includes a pad having a pad surface for at least intermittently contacting the surface of the substrate. The apparatus also includes a first electrode, such as a working electrode in a potentiostat system, operable to contact the surface of the substrate, and a second electrode, such as a reference electrode in a potentiostat system, operable to contact the electrolytic solution. A voltage sensing circuit, such as a potentiostat, is coupled to at least the first and second electrodes for sensing a change in the electrochemical potential between the first and second electrodes.
A preferred embodiment of the apparatus includes a controller coupled to the potentiostat. The controller generates a process control signal for stopping the planarization process based on a substantial increase in the electrochemical potential between the first and second electrodes as indicated by the potentiostat.
By measuring the electrochemical potential between the surface of the substrate and the electrolytic solution during a planarization process, the present invention provides a more accurate indication of the time at which the conductive material is completely removed than was previously available. Thus, implementation of the invention substantially reduces the probability of removing too much or too little material during the planarization process.
In another aspect, the invention provides a method for controlling a process, such as an electrochemical planarization process, wherein a thickness of a conductive material, such as copper, is altered on a surface of a substrate. Electrical continuity is established between the surface of the substrate and a first electrode, such as a working electrode of a potentiostat system. The surface of the substrate is brought into contact with an electrically conductive solution, and electrical continuity is established between the solution and a second electrode, such as a reference electrode of the potentiostat system. The method includes measuring a value of an electrochemical property, such as the electrochemical potential, between the first electrode and the second electrode, and interpreting the value of the electrochemical property as a measure of the electrical conductivity of the surface of the substrate. The planarization process is then controlled based on the value of the measured electrochemical property.